Injection molding system with sequential gate control

ABSTRACT

An injection molding machine [10] for molding plastic parts by injecting a molten plastic through a plurality of valve gates [31] into a mold cavity by reciprocation of a helical screw [16]. A linear position sensor [34] positioned on the machine to detect the position of the helical screw [16] and control the opening and closing of the gates [31] based on the detected position of the screw [16]. A computer is in communication with the valve gates and the position sensor to monitor the position and activate the opening and closing of the valve gates [31]. A pressure transducer is associated with the mold cavity and in communication with the computer [32] as an alternate or additional method for controlling the opening and closing of the valve gates [31].

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. Ser. No. 08/934,170, filed Sep.19, 1997 now U.S. Pat. No. 5,919,492 and claims benefit from prior filedco-pending provisional application Ser. No. 60/049,531 filed Jun. 13,1997.

TECHNICAL FIELD

The present invention relates to a method and apparatus for injectionmolding. More particularly, the present invention relates to a methodand apparatus for injection molding parts by sequential control of valvegate bushings to regulate the flow of a molten material into a moldcavity to form a molded part.

BACKGROUND OF THE INVENTION

Injection molding machines and certain processes for injection moldingare well known in the art. According to known injection molding methods,a molten material, such as plastic, is distributed to a mold having twohalves which are then closed upon one another to form a part in theshape of the cavity that is formed in the mold.

More specifically, in known injection molding methods and processes, themolten material is supplied to the mold from an injection moldingmachine. The material is fed into the machine via a hopper or the like.The material is then transferred to a barrel associated with the machinewhere it is heated. The barrel houses a reciprocating screw whichreciprocates linearly therein. During the injection molding process, thereciprocating screw moves toward the mold forcing the molten materialthrough a manifold and then through a plurality of bushings into themold cavity or cavities. The bushings are placed in the mold, at variouslocations depending upon the part to be molded.

It is necessary to control the amount of molten material that issupplied to the mold cavity, as the required amount depends on the size,shape, thickness and other factors of the part or parts to be molded.The amount of molten material that is supplied through each bushing istypically controlled by opening and closing each of the valve gatesbased on time control. The time at which the valve gate bushings openand close and the duration that they are open and closed ispredetermined by the needs of the particular part to be molded and typeof material being used. These are typically predetermined throughexperimentation, which takes a considerable amount of time.

For conventional injection molding methods utilizing a timer control,the opening period of the valve gates can only be controlled typicallyon the order of 0.01 seconds. Thus, due to various uncontrolled factors,such as changes in the flow resistance due to temperature and/ormaterial viscosity changes, the amount of molten material flowingthrough the valve gates during a certain period of time cannot beaccurately controlled. As a result, it is known that some rejects willbe produced.

One known method is disclosed in U.S. Pat. No. 5,389,315. In thissystem, the valve gates of the bushings are sequentially opened oneafter the other based on the amount of molten resin supplied from thebarrel as detected by metering mechanisms associated with each of thevalve gates. Additionally, the patent discloses opening and closing thevalve gates without any overlap based on the position of an in-linescrew.

As the valve gates are opened and closed without any overlap, thequality of the part can be affected, and the time to manufacture such apart can be increased.

Further, the type of parts that can be made with such a process can belimited.

These prior methods are basically unable to consistently monitor andcontrol the amount of material injected into the mold cavity withsufficient accuracy to produce a high quality molded part. Further,these methods and apparatuses require prolonged set up times and resultin waste of material during the molding start-up.

SUMMARY OF THE INVENTION

It is an object of the present invention to measure and control theamount of plastic or other molten material flowing through the manifoldand a plurality of valve gate bushings by opening and closing the valvegates based on an indication of the position of a reciprocating screw ofan injection molding machine as represented by a separate linearpotentiometer attached to the machine.

It is a related object of the present invention to measure and controlthe amount of plastic or other molten material flowing through amanifold and a plurality of valve gate bushings by opening and closingthe gates based on the cavity pressure in the mold cavity.

It is another related object of the present invention to measure andcontrol the amount of plastic or other molten material flowing through amanifold and a plurality of valve gate bushings by opening and closingthe valve gates based on time, linear movement of the screw, and/orpressure, or any combination thereof.

In accordance with the objects of the present invention, an injectionmolding machine comprises a material hopper for receiving material to beused in the molding process. The material is heated to a molten state inthe barrel. The barrel houses a reciprocating screw which axiallyreciprocates therein. The initial position of the screw is detected aswell as the absolute shot size required to mold the part by anexternally mounted linear potentiometer. The number of bushings in theinjection mold is also entered into the controller which monitors andcontrols their opening and closing and the operation of the valve gatesystem. The bushings have valve gates which are controlled by pinmembers to open and close the valve gates to respectively allow andrestrict the flow of material into the mold cavity. The controllerdetects when the mold is closed before allowing the injection moldingprocess to commence. The remainder of the mold filling and packingprocess is computer controlled.

The controller is also in communication with the linear potentiometer orposition sensor to monitor the position of the reciprocating screw. Thecontroller can also be placed in communication with a pressuretransducer to monitor the pressure in the mold cavity.

Based on receiving the mold closed signal from the injection moldingmachine along with the system ready signal from the controller to theinjection molding machine, the reciprocating screw, under pressure,forces molten material into the mold. The opening and closing of thevalve gate pins allows the flow of molten material into the moldcavities. This is controlled by the PC based computer system based onthe position of the screw as determined by the linear potentiometer, thepressure in the mold cavity as determined by the pressure transducer,and/or time.

Additional features and advantages of the present invention will becomeapparent to one of skill in the art upon consideration of the followingdetailed description of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view illustration of an injection molding machine inaccordance with a preferred embodiment of the present invention;

FIG. 2 is a cross-sectional view illustrating the communication betweenthe barrel and the mold cavity in accordance with a preferred embodimentof the present invention;

FIG. 3 is an illustration of a bushing, including a valve gate and pin,in accordance with a preferred embodiment of the present invention;

FIG. 4 is a bar chart illustrating a mold fill sequence used to controlthe opening and closing of a plurality of valve gates based on a timerto mold a particular part;

FIG. 5 is a bar chart illustrating a mold fill sequence used to controlthe opening and closing of a plurality of valve gates based on linearscrew position to mold a particular part;

FIG. 6 is a diagram illustrating the system configuration which definesthe input of devices or parameters to control the valve gates inaccordance with the present invention;

FIG. 7 is a diagram illustrating the valve gate configuration to beentered for each specific valve gate in accordance with the presentinvention;

FIG. 8 is a diagram illustrating the system status in manual controlallowing opening and closing of the individual gates on setup prior tocommencement of the molding process;

FIG. 9 is a diagram illustrating the system status prior to operation ofthe molding process in accordance with the present invention;

FIG. 10 is a diagram illustrating the system status after completion ofa molding process in accordance with the present invention;

FIG. 11 is a diagram illustrating the verify home position screen for asingle reciprocating screw in accordance with the present invention;

FIG. 12 is a diagram illustrating the valve gate configuration screenfor a dual reciprocating screw system in accordance with a preferredembodiment of the present invention;

FIG. 13 is a diagram illustrating the verify home position screen forthe dual reciprocating screw system in accordance with a preferredembodiment of the present invention; and

FIG. 14 is a diagram illustrating the system status prior to operationof the molding process for the dual reciprocating screw systemembodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 illustrates an injection molded machine 10. The machine 10 isused in the injection molding process to service a mold which formsmolded parts. The molded parts are preferably plastic.

However, it should be understood that the parts can be molded from anysuitable material.

The machine 10 is part of an injection molding system and includes amaterial hopper 12 that receives the material to be molded in anon-molten form. Such material is preferably in the form of pellets. Thepellets are fed into the hopper 12 and are transferred directly to abarrel 14 associated with the machine 10. The pellets are heated to amolten state by electric heaters or the like (not shown) on the barrel14. The barrel 14 includes a reciprocating screw 16 reciprocatingtherein to inject the molten material into the mold. The reciprocatingscrew 16 also acts as a piston and can be pressurized to drive themolten material to the mold.

The machine 10 further includes a motor 18 which drives thereciprocating screw 16. The injection molding machine 10 has a pair ofmachine platens 23, 25. A pair of mold halves 20, 22 (FIG. 2) are eachpositioned on respective platens. The first platen 23 is stationarywhile the second platen 25 is moveable by a motor to move the moldhalves 20, 22 into contact with one another. The first mold half 20 isin communication with the barrel 14, while the second mold half 22 ismoveable toward the first mold half 20 into a closed position wherebythe two mold halves 20, 22 form a mold cavity 24, in the shape of thepart to be molded.

The barrel 14 is in communication with the first mold half 20 and thusthe mold cavity 24 at a first end 26. The molten material is forced bythe movement of the reciprocating screw 16 through the first end 26 ofthe barrel 14 and into a manifold with a plurality of passageways 28.The passageways 28 in turn are in communication with the manifold and aplurality of valve gate bushings 30 placed in the first mold half 20.The reciprocating screw 16 acts like a piston and as it moves toward themold, it carries molten material forward therewith.

The number of valve gate bushings 30 and their location on the surfaceof the first mold half 20 may vary depending upon the size and shape ofthe part or parts to be molded. Thus, the number of passageways 28 inthe manifold will also increase proportionally. The computer 32, such asa Pentium equipped computer, allows for user interface allowing a userto input information. However, the computer 32 can be any commerciallyavailable computer. In the preferred embodiment, an I/O board, a timingboard, a driving board, and an analog board are also integrated into thecomputer 32. These boards are all commercially available. The computer32 also preferably includes a commercially available touch screen orflat panel display.

FIG. 3 illustrates a valve gate bushing 30 adapted to receive moltenmaterial from the barrel 14. The valve gate bushing 30 has a valve gate31 that opens into the mold cavity allowing molten material to flowtherethrough. A pin 33 is also included which moves back and forth toopen and close the valve gate 31 and thus control the flow of moltenmaterial therethrough. The operation of these bushings 30 is well knownin the art.

FIG. 4 illustrates a valve gate controller timing chart for controllingthe opening and closing of a plurality of bushing valve gates 31 basedon time as regulated by the computer 32. The respective valve gates 31are illustrated on the vertical axis of the chart while the travel ofthe reciprocating screw 16 in time (seconds) is represented on thehorizontal axis. The bar chart in FIG. 4 illustrates the mold fillsequence for a particular part. It is known in the art that the sequencein which various valve gates are opened can be varied to mold the partor parts more effectively. Also, weld lines can be eliminated or movedto a non-critical area of the part by varying the sequence in which thevalve gates are opened and closed.

As shown in FIG. 4, eight bushings numbered one through eight arelocated on the surface of the mold half 20. However, it is obvious thatany number of valve gate bushings 30 can be used. The computer 32 isprogrammed with the required information on the volume required for thepart or parts to be molded. While the system, shown in FIG. 4, hastwelve valve gates, only gates one through eight are used in thisexample. Thus, during set up of the computer 32 prior to initiating theprocess, gates one through eight are programmed to be "on" while gatesnine through twelve are programmed to be "off" since only eight bushingsare used in this example. The computer 32 sends a signal to the machine10 indicating that the system is ready. Then the mold halves 20, 22close and a signal is sent to the computer indicating that the mold isclosed and the injection process is initiated. The reciprocating screw16 under pressure moves towards the mold halves 20, 22 forcing moltenplastic toward the valve gate bushings 30, which open in accordance withthe set valves. The molten material travels through the open valve gatesand into the mold cavity 24 to form the part. The valve gates, pneumaticor hydrantic, are preferably solenoid actuated.

As shown in this embodiment, where the valve gates open and close basedsolely on time, at time 0.00 valve gate numbers one and five are openedallowing molten plastic to flow therethrough. As the screw continues tomove, at a point just after 0.00 seconds, approximately 0.40 seconds,valve gate number six opens. After 3.6 seconds of total screw traveltime, valve gate number one closes. Valve gate number one was thus openfor 3.6 seconds. Valve gate number two opens after about 4.20 seconds ofscrew travel time, while valve gate number four opens after 5.0 secondsof screw travel time and closes at the same time that valve gate numberseven opens after about 5.4 seconds of screw travel. Valve gate number 4was thus open for 0.40 seconds.

After 5.8 seconds, valve gate number five closes and after 6.0 secondsof screw travel time, valve gate number six closes, preventing anyfurther flow of molten material therethrough. Valve gate number five wasopen for 5.8 seconds and valve gate number six was open for 5.6 seconds.Valve gate number seven closes after 10.0 seconds of screw travel timeand was open for 4.6 seconds, while valve gate number two closes after8.2 seconds and was open for 4.20 seconds. After 8.0 seconds of screwtravel time, valve gate number three opens and then closes one secondlater after 9.0 seconds of screw travel time. After 14.6 seconds, valvegate number eight closes after being open for 7.40 seconds and the moldfill sequence is complete. It should be understood that all of thesetimes are approximations. As discussed above, because of variances withthe screw 16 as well as the valve gate bushings 30, opening and closingthe valve gates 31 based on time can result in an unevenness in the moldfilling process and usually results in a reject because of too much ortoo little molten material being injected into the mold. Over the courseof molding many parts, this is a significant expense.

As shown in FIG. 1, the injection molding machine of the preferredembodiment, also includes a linear transducer 34. The linear transducer34 or linear potentiometer is in communication with the computer 32 tomonitor the position of the reciprocating screw 16. The lineartransducer 34 is comprised of an encoder that is attached to the machine10 and produces an output to indicate the position of the screw 16.

Thus, in accordance with the preferred embodiment of the presentinvention, the opening and closing of the valve gates 31 can be alsocontrolled by the computer 32 based solely on the position of thereciprocating screw 16.

Alternatively, a pressure transducer (not shown) can be included tomonitor the pressure in the mold cavity 24 by being placed directly inthe mold cavity 24 or at the entrance to the valve gate bushings 30 tomeasure the pressure created by the molten material in the cavity 24.The pressure transducer sends an electrical output signal to thecomputer 32 to control the system in accordance with the inputparameters.

FIG. 5 illustrates a mold fill sequence similar to that shown in FIG. 4.However, the mold fill sequence of FIG. 5 is based on the linear screwposition, as indicated by the linear transducer 34, instead of by time.As shown in FIG. 5, prior to initiation of the injection moldingprocess, the computer 32 is programmed such that valve gate numbers onethrough five and seven and eight are turned "on" as they will beutilized in the process while valve gate numbers six and nine throughtwelve are turned "off" as they are not used in the process for thisparticular part.

Once the computer 32 receives the signal that the mold halves 20, 22 areclosed and the system is ready, the injection molding process isinitiated. Based on the length of screw travel, valve gate numbers onethrough five and seven and eight open and close in response to certainpositions of the reciprocating screw 16 as detected by the lineartransducer 34.

FIG. 5 is a graph illustrating the valve gates 31 on the vertical axisand the screw travel in inches on the horizontal axis. After initiationof the injection molded process, the screw moves from its startingposition (20.0 inches). Immediately after initiation of the process at19.99 inches, valve gate number four opens. Valve gate number fourcloses at 14.64 inches (5.35 in. of screw travel). At 14.8 inches valvegate number one opens, and at 12.26 inches valve gate number threeopens. Valve gate number one closes at 12.2 inches (2.6 in. of screwtravel). Valve gate number two opens at 9.68 inches while valve gatenumber three closes at 9.52 inches (2.74 in. of screw travel). Valvegate number five opens at 6.78 inches while valve gate number two closesat 6.62 inches (3.06 in. of screw travel). Valve gate number seven opensat 4.64 inches, valve gate number five closes at 4.48 inches (2.30 in.of travel). Valve gate number eight opens at 2.84 inches, and valve gatenumber seven closes at 2.2 inches (2.44 in. of screw travel). Valve gatenumber eight closes at 0.0 inches (2.84 in. of screw travel). Openingand closing of the valves by linear position of the reciprocating screw16 is far more accurate than opening and closing them based on time.Setup time can also be decreased.

Turning now to FIGS. 6 through 10 which illustrate a preferredembodiment of the computer controlled sequencing of the valve gatebushings 30 in accordance with the present invention. FIG. 6 illustratesthe configuration of the system which is set up prior to commencement ofthe injection molding process. During set-up, the user can selectwhether the mold fill sequence is to be based on time, position of thescrew 16, and/or pressure or a combination thereof.

While only eight gates are shown being used in FIGS. 4 and 5, it is thusobvious that more or less valve gate bushings 30 may be employed andthat the valve gates 31 may open or close at different times and/orpositions. Additionally, the various parameters can also be changeddepending upon the part to be molded. FIG. 6 illustrates the preferredparameters for one embodiment of the present invention. The data inFIGS. 6 through 10 is all preferably input into the computer 32 via atouch screen. Alternatively, a mouse or a keyboard, can be used to inputinformation. However, other known apparatus for data entry may be used.Further, if the operator intends to mold a part based on previously setup parameters, such parameters can be pulled up from a previously savedfile and used without any further set up and without the need tore-input the parameters as the parameters from the file will be used.The system can also be used to convert a file with stored parametersbased on time to parameters based on screw travel. This is an advantagefor a machine that previously ran based on time, but is being equippedor retrofitted to run based on screw position. The preferred system alsoallows for conversion of the parameters from screw position to time.

The maximum screw travel distance is entered in box 42 and is shown as15 inches. The allowable set for the home position or starting point ofthe screw is 0.0625 inches, shown in box 44. The maximum cycle time is25 seconds as shown in box 46. Further, the system can be toggled tooperate in metric by entering millimeters instead of inches into box 48.These parameters can all be varied or adjusted depending upon theinjection molding apparatus. The box 40 indicates the output of thetransducer which in the preferred embodiment is 10 V, but this may alsovary.

FIG. 7 illustrates the gate configuration for the valve gate bushingnumbers one through four to be used in connection with molding a part inaccordance with the parameters input into the computer 32. However, thesystem may be upgraded to forty or more valve gates as necessary. Eachvalve gate bushing 30 has a button 50 that allows the operator to toggleit "on" or "off". As shown, valve gate bushing numbers one through fourare all in the "on" position. Each valve gate 31 has at least twocolumns associated therewith indicating the capability for each valvegate to open and close more than once during any given injection moldingprocess. For example, valve gate one opens and closes twice in this oneprocess.

Valve gate number one is opened the first time at 7 inches as shown inbox 52 and closes at 6 inches (box 54). Valve gate number one opens asecond time at 5.5 inches as shown in box 56 and closes at 5.0 inches asshown in box 58. Valve gate number two opens at 6 inches (box 60) andcloses at 5 inches (box 62), while valve gate number three opens at 5inches (box 64) and closes at 4 inches (box 66), and valve gate numberfour opens at 3 inches (box 68) and closes at 0.25 inches (box 70). Thepositions in the various boxes can be modified as needed by the operatorto change the opening and closing positions of each of the valve gates.The positions of the remaining vale gates, if necessary, are notspecifically shown in this figure and will vary depending upon the partto be molded. In the preferred embodiment, the screw positions aremeasured based on a predetermined screw starting point and as the screwtravels it moves toward a zero ending position. A disable button 72 isalso included to open all gates while the mold is open. This processwarms the pin and is preferred in some molding applications such as forheat sensitive materials.

FIG. 8 illustrates a manual operator screen that allows an operator toopen gates manually while watching the linear travel of the screw. Thisscreen, like the others, indicates the position of the mold halves 20,22. The liner travel of the screw is represented by the bar 74, which isbased on the information received from the linear transducer. A statuscolumn 76 is also present indicating various system inputs and outputs.The system input is a mold closed indicator 78 that tells the computer32 that the mold is closed. System outputs include an alarm light 80, asystem ready light 82, and an emergency stop 84 which can all be testedby operator activation.

FIGS. 9 and 10 illustrate the sequence of opening and closing the valvegates. This screen is displayed throughout the molding cycle and itcontinuously changes so that it can be monitored by an operator. Thelinear device data is shown on bar 86. This data includes the maximumvoltage output of the transducer, the maximum travel distance of thescrew, the units (inches or millimeters) the system is operating in, theallowable offset from home, and the maximum cycle time. The status bar88 illustrates the current system status. The home position or startingpoint of the screw is shown at 90, the present distance of travel forthe screw is shown at 92, and the time elapsed is shown at 94. Thestatus bar 88 also indicates the status of the system inputs (moldclosed) 78 and system outputs (alarm) 80, (system ready) 82, and(emergency stop) 84.

The valve gate status is shown in the box 100 on the left-hand portionof the screen which indicates the valve gate number 102, the action ofthat valve gate 104 (whether it opened or closed), the travel of thescrew at which the action took place 106, and the time at which theaction took place 108. As the screw position changes, the status ofthese valve gates change and the information appears on the box as thescrew travels from its home position throughout its set length oftravel. FIG. 10 illustrates the status of the system after a full cycleand thus all the valve gates are closed. There are a variety ofintermediate screens that appear throughout a single process, as can beunderstood by those of skill in the art. After the mold closes, the box100 is cleared, the screw then returns to its home position, and theprocess is again ready for initiation to mold another part.

FIG. 11 illustrates a verify home position screen. This allows theoperation to ensure that the reciprocating screw 16 is in the homeposition prior to initiating the injection molding cycle. If the screwis not at the home position or within its allowable offset, a rejectpart will be formed as either too much or too little material will bedelivered to the mold. If the screw is in the proper position, theoperator will click the button 120 to continue with the set-up. Theposition of this reciprocating screw is show in box 130 so its positionmay be viewed.

Each of the valve gate bushings 30 may include a sensor associatedtherewith. The sensor provides feedback to the computer 32 as to anyaction taken by that valve gate, i.e, whether the respective valve gateopened as required or not. If the valve gate did not open as required,the pressure could build up and damage the mold. The alarm will beactivated if one or more of the valve gates did not open as required orotherwise failed. This provides constant feedback that the system isoperating as required.

In operation, the computer 32 receives a signal from an encoder, alinear potentiometer, or a linear motion transducer. The computer canalso receive a signal from a pressure transducer, a timer, and atemperature control input located inside the mold cavity that monitorsthe surface temperature of the mold. Additionally, feedback can beprovided to the computer 32 to confirm that water is on and to ensurethat an overheat condition is not achieved in the mold. The computer 32also receives signals from the injection molding machine that indicateswhether the mold is in the open or closed position.

As the computer 32 is in communication with, but separate and distinctfrom the machine 10, the computer 32 and inputs such as keyboard,screen, and mouse can be located at a position distant or remote fromthe machine. Thus, the operator can be located at a remote position andcan control the machine processes via wires, fiber optics, ISDN lines,or even by modem. Additionally, files containing setup information andparameters can be downloaded from one computer to another to furtherdecrease the set up time.

An alternative embodiment of the present invention is shown in FIGS. 12through 14. In this embodiment, two screws are used to perform theinjection molding of a part or parts and two linear potentiometers ortransducers are included on the machine. The first linear potentiometerposition monitors the first screw and the second monitors the secondscrew. The use of two screws is useful in a variety of operations. Inone embodiment, wherein 12 valve gates are needed to form a part, thefirst reciprocating screw can be used to supply molten material to valvegate numbers one through five. The second reciprocating screw can beused to supply molten material to valve gates six through twelve.

In another embodiment, the reciprocating screws can each be used to molda separate part. For example, the screws can be set up to mold twomating parts at the same time. For example, one screw can be used toprovide molten material to a manifold and a plurality of valve gates andto a mold cavity to mold a cup while the second screw can be used toprovide molten material to a separate manifold and a separate pluralityof valve gates and then to a separate mold cavity to form a mating topfor the cup. In yet another embodiment, the reciprocating screws can beused to mold a part where one reciprocating screw provides moltenmaterial to form a skin for a part while the second reciprocating screwprovides molten material to form a core for the same part. It should beunderstood that the screws operate as discussed above. Further, it isobvious that more than two screws and thus more than two lineartransducers can be used. Further, one screw can operate to open andclose the gates based on screw positions, while the second screw can beused to open and close the gates based on time.

FIGS. 12 through 14 illustrate one embodiment of how the computer andsystem can be set up to operate two reciprocating screws to mold a partor parts. FIG. 12 illustrates the gate configuration for a dual lineardevice system. This figure illustrates a screen for configuring valvegate numbers one through four. However, as discussed above, 40 or morevalve gates may be utilized depending upon the part to be molded.

Valve gate numbers 1 through 4 are all illustrated as being "off", butby activation of the buttons 200, each of the valve gates can be toggledto be "on" for operation in the next injection molding cycle. A lineardevice button 202 is included to tell the computer 32 to whichreciprocating screw the valve gate will be connected. The button 202 canbe toggled from a "0" for the first reciprocating screw to a "1" for thesecond reciprocating screw. Therefore, if the gate button 200 is on, thecomputer 32 will check the status of button 202 to see which screw iscontrolling the valve gate.

The screw position at which the valve gates are to open is shown at 204,while the position at which the valve gates are to close is shown at206. As with the single linear device system, a pair of columns 208, 210are present allowing each gate to open more than once during a givenrun. The units of measurement is shown in inches, but can be toggled bybutton 212 to be measured in millimeters.

FIG. 13 illustrates the verify home position screen. This allows theoperator to ensure that both reciprocating screws are in the homeposition prior to initiating the molding cycle. If the screws are in theproper positions, the operator will click button 220 to continue withset up. The position of the reciprocating screws is shown in boxes 222and 224 respectively so the operator may view their respectivepositions.

FIG. 14 illustrates the execute gate sequence for a dual linear system.The bar 230 illustrates the parameters of the first reciprocating screw.Specifically, the output of transducer (232), the maximum travel of thescrew (234), the units of measurement (236), the offset from the homeposition (238), and the maximum cycle time (240). The bar 242illustrates the same parameters as shown on the bar 230, but for thesecond reciprocating screw.

The current status is shown for the first reciprocating screw on bar 244while the current status of the second reciprocating screw is shown onbar 246. The display 248 illustrates the opening and closing of thegates as they occur based on screw travel or time while the status ofthe gates at any given time is shown at 250.

While only one preferred embodiment of the invention has been describedhereinabove, those of ordinary skill in the art will recognize that thisembodiment may be modified and altered without departing from thecentral spirit and scope of the invention. Thus, the embodimentdescribed hereinabove is to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims, rather than by the foregoingdescriptions, and all changes which come winded to be embraced herein.

What is claimed is:
 1. A method for injection molding a plastic part,including an injection molding machine, having a barrel filled with amolten material and a reciprocating screw therein, the barrel in fluidcommunication with a pair of mold halves, the method comprising:closingthe pair of mold halves to form at least one mold cavity in the shape ofthe part or parts to be molded; positioning a plurality of normallyclosed valve gates across the surface of one of the pair of mold halves;monitoring the position of the reciprocating screw as it moves through alength of travel from a home position to an end position; programming acomputer associated with the machine to open and close said plurality ofvalve gates during a single length of travel of the reciprocating screw,said plurality of valve gates in communication with said computer toeach open and close based solely on the position of the screw as itmoves through said length of travel; positioning the screw in said homeposition based on the part or parts to be molded during a given moldingcycle; and moving the reciprocating screw from said home position tobegin said given molding cycle, said movement causing said computer toopen at least one of said plurality of valve gates based on the linearposition of the screw, allowing molten material to flow into said atleast one mold cavity through said at least one open valve gates; andcontinuing to move the reciprocating screw toward said end position,said movement causing said computer to close said at least one openvalve gate based on the linear position of the screw preventing moltenmaterial from flowing therethrough, said continued movement causing saidcomputer to open another of said plurality of valve gates based on thelinear positions of the screw allows molten material to flow throughsaid another open valve gate.
 2. A method for injection molding aplastic part as recited in claim 1, wherein each of said plurality ofvalve gates are opened simultaneously.
 3. A method for injection moldinga plastic part as recited in claim 2, wherein each of said plurality ofvalve gates are closed at different screw positions.
 4. A method forinjection molding a plastic part as recited in claim 1, wherein each ofsaid plurality of valve gates are opened at different screw positions.5. A method for injection molding a plastic part as recited in claim 1,further comprising:opening and closing at least one of said plurality ofvalve gates more than once during said given molding cycle.